- 
Modern Chimp Skull vs. 7 Million year old skull
Must be one of Janet Reno's ancestors...
Question: How can this be 2002 and we still have adults who still don't know the answer to this question?
Doesn't it seem that in seven million years we should have advanced further than we have.
Brian.
NATURE: the British heir to the Soviet Academy of Science. Subscribe only if you are house-training a DOG. HOO-HAA.
Sure they can. You can make "family" connections based which traits are present and which are not. For example, a particular bony ridge with certain unique characteristics might be present in one lineage (since they descended from a common ancestor which first developed that unique feature, whereas a lineage which diverged earlier than the appearance of the bony ridge would not have that feature.
Using that one feature, and dozens (often hundreds) of others which are characteristic enough to be distinguished from non-related features, it's almost child's play to draw the family tree by tying together thsoe which had to have common ancestors before/after the development of a given feature.
There are of course some things that make it tricky sometimes, like making sure that an apparent shared feature is actually the same feature and not just two features that share some similarities. For example, both humans and squids have eyes, but that doesn't mean that their very distant common ancestor (*WAY* back when) had eyes which were passed down to both lines. Actually, the molluscs developed eyes independently of the vertebrates -- but although both are "eyes", even a layman can see from looking at them that the mollusc eye and the vertebrate eye share only superficial similarities (forced by function) but all the details are quite different and they're really not the same type of eye at all. So although it takes some examination to determine what is an actual shared feature, and what is only a similar feature found in two places, it's not really that hard to tell one category from the other.
Also, the "shared feature" linkage can't distinguish between (as the article terms it) "grandfathers" and "grand uncles". For example, imagine a family where Pappy Jones was a mutant born in 1825 with a unicorn-like horn on his forehead. All of his children, and their children, and so on inherited the horn also. All the family's geneological information got lost in the fire of 1969, unfortunately, so when a horned skeleton is found in an Ohio cave, and another is later found at the bottom of an old well in Florida, there are a number of things you can and can not safely conclude once you verify that the horns on the skeletons are definitely the Pappy Jones sort of horns:
1. The skeletons are definitely both part of the Jones family.
2. They're definitely descendants of Pappy Jones himself, and aren't just in the Jones family via Pappy's brother's descendants, etc.
3. If Susie Jones is alive and well today (horn and all), the two skeletons are definitely in her near family tree (tied to her through Pappy), but may or may not be her direct great-great-great-etc.-grandfather(s).
4. The skeletons are definitely *not* more closely related to hornless Joneses than they are to horned Joneses. Any living hornless member of the Jones family has to have be related to the skeletons more distantly in the past than Pappy himself. In other words, if they share a common ancestor, it's one of Pappy's ancestors, not Pappy himself or any of his descendants.
5. Note that you can safely make all these conclusions without being able to tell how many generations separate the skeletons from Suzy or Pappy, nor which line of horned Joneses they're actually directly linked to, nor even anything about the "missing link" family members.
This is how fossils are linked into "family" trees of species, and how living species are linked into family trees of close relatives, farther relatives, and distant relatives. For example, we can know for a biological fact (even aside from the obvious visual similarities) that we're much more closely related to the great apes than we are to racoons, are closer to racoons than to birds, are closer to birds than to lobsters, and are closer to lobsters than we are to redwood trees.
And when it comes to documenting family trees of living species, we can use far, far more than just skeletal similarities (which is often all we have to go on with fossils) -- there are a myriad of chemical and DNA features that can be used to create a very highly accurate map of common ancestries.
The work of tying together the millions of living species on Earth (much less the extinct ones discovered through fossils), and then similarly tying together the genuses, the families, the orders, and the higher groupings, is a daunting task that will take a very long time, but you can browse one such work in progress at the Tree of Life Web Project.
Question: How does one species beget another?
Imagine a population of species X that lives in the forest. As it grows and prospers, it spreads out geographically. Consider some different cases:
Case 1: A lightning strike starts a forest fire, and hundreds of acres are burned. After it burns out, there are a group of survivors of species X to the west of the burned region, and another group of survivors to the east. They resume life as usual, but because of the burnt region, where there are none of the trees that they need, the western population and the eastern population never cross the charred area to meet up and reconsolidate the species population. They are now two totally independent populations. Over hundreds of generations, the western population may have evolved new features to better survive and prosper, and the independent eastern population may have done likewise, but would almost certainly have evolved *different* features than the western population. Being physically separated, they are each free to follow their own evolutionary path, and as changes accumulate in each population, they will become more and more different from each other. This is one way that a single species can *fork* into two or more species.
Case 2: While expanding geographically, some of species X spreads up the slopes of a mountain, while others of the same species spread down into a valley. Over many generations, the population on the mountain will adapt to better prosper in their mountainous environment. Similarly the valley-dwellers will adapt to valley life. Eventually, over hundreds of generations, they can reach the point where the mountain population can be quite different from the valley population. Given enough time, they'll be enough different that they won't even interbreed, for any number of reasons (behavioral incompatibility, or one type will no longer appeal sexually to the other, or they will vary enough in size to be mechanically unfit for mating, or the mountain group will have acquired instincts to not leave the mountain, or one group will have migrated too far away from the other to ever meet up again, etc. etc.) At this point they are two distinct species, both born of the same original forest-dwelling species.
The short form is that a single species can stay pretty much the same over time (the horseshoe crab hasn't changed much in a *long* time), or it can change over time without spinning off "sister" species, or subpopulations of the species (separated by geographic distance, or barriers, or habitat differences) can split apart and each go their own way.
Darwin was struck by how the various types of finches in the Galapagos islands seemed to be related, but differed in striking ways, depending on habitat, geographic separation, or the gulfs between the islands, and that the more separated they were, the more they differed. This is what led him to conceive of the notion of common ancestry and change over time -- the "family tree" of the finches, and other animals, jumped out at him. And when he started considering other "tree of life" relationships, the same pattern showed itself:
When on board H.M.S. Beagle, as naturalist, I was much struck with certain facts in the distribution of the inhabitants of South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to me to throw some light on the origin of species -- that mystery of mysteries, as it has been called by one of our greatest philosophers. On my return home, it occurred to me, in 1837, that something might perhaps be made out on this question by patiently accumulating and reflecting on all sorts of facts which could possibly have any bearing on it. After five years' work I allowed myself to speculate on the subject, and drew up some short notes; these I enlarged in 1844 into a sketch of the conclusions, which then seemed to me probable: from that period to the present day I have steadily pursued the same object. I hope that I may be excused for entering on these personal details, as I give them to show that I have not been hasty in coming to a decision.
....never mind.
redrock
One species doesn't exactly beget another. Another species happens when one group that is closely related to another STOPS begetting with the other group. Beagles and greyhounds are both varieties of the same species. If one of them changed so drastically that its offspring could no longer reproduce with the other they would then be two distinct species.
Mr. Brunet was later quoted as saying: "Now, if I could just find my $@&*$@%$ car keys!!"
- Modern Chimp Skull vs. 7 Million year old skull